The present invention relates to an electric apparatus containing insulating oil. More particularly, it relates to a transformer having a forced oil cooling system in which the generation of static electricity can be prevented with the forced flow of an insulating mineral oil. More particularly, a charge of static electricity produced by the flow of an insulating oil is prevented by modifying a surface of an insulating sheet in contact with the insulating oil by using a material which imparts static electricity having the opposite polarity.
When insulating oil is in contact with the surface of a solid, positive and negative charges in the oil are separated to form a double layer of the charges at the solid-liquid interface. When flow of the insulating oil occurs at the interface, the charge layer in the oil is conveyed by the flow of the oil and is separated from the charge on the surface of the solid which is of the opposite polarity. The amount of charge built up by the process is dependent on the rate of the charge separation and the rate of the relaxation of the charge to the ground. The rate of charge separation is higher for a lower value of resistivity of the insulating oil, while the relaxation of the charge decreases with the resistivity of the insulating oil. As a result, the charge produced by the flow of oil is highest in the range of 10.sup.12 to 10.sup.14 ohm-cm of volume resistivity of the insulating oil.
Most insulating mineral oils used for electric apparatus are included in the range of the volume resistivity so that the insulating mineral oils have a tendency to produce a high charge build-up. When the static electricity charge in the insulating mineral oil or on the solid exceeds a certain critical level, a discharge occurs which may cause failure of the apparatus.
Heretofore, it has been proposed to decrease the volume resistivity of oil to a value lower than 10.sup.12 ohm-cm by adding an ionic additive to prohibit the explosion hazard caused by the static charge build-up in the petroleum industry. However, it is not desirable to decrease the volume resistivity of oil since it causes inferior electric characteristics. From an industrial viewpoint, it is difficult to increase the volume resistivity of the insulating mineral oil to a value higher than 10.sup.15 ohm-cm purifying the insulating mineral oil in order to realize a volume resistivity which is not in the range of 10.sup.12 to 10.sup.14 ohm-cm.
Even though an insulating mineral oil having a volume resistivity of higher than 10.sup.15 ohm-cm is obtained at a specified temperature, the volume resistivity is easily decreased by deterioration or simply by a rise in temperature.
Accordingly, it is not desirable to control the volume resistivity of an insulating mineral oil for electric apparatus to prevent electrostatic charge build-up.